06 Add an Overlay to an Interactive Map

Using the sf and tmap packages to overlay a spatial object ontop of a leaflet map with an OS Maps API base map.

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tmap

The tmap R package is providing a wrapper around the JavaScript web mapping library Leaflet.js. It allows the visualisation of geospatial data in R on an interactive leaflet map.

library(sf)

Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1; sf_use_s2() is TRUE

library(tmap)

Registered S3 methods overwritten by 'htmltools':
  method               from         
  print.html           tools:rstudio
  print.shiny.tag      tools:rstudio
  print.shiny.tag.list tools:rstudio
Registered S3 method overwritten by 'htmlwidgets':
  method           from         
  print.htmlwidget tools:rstudio
The legacy packages maptools, rgdal, and rgeos, underpinning this package
will retire shortly. Please refer to R-spatial evolution reports on
https://r-spatial.org/r/2023/05/15/evolution4.html for details.
This package is now running under evolution status 0 

Create sf object from GeoPackage (GPKG)

# Create an sf data.frame object from the Greenspace file
osogs <- st_read('../../data/ordnance-survey/os-open-greenspace-gb.gpkg',
                 layer = 'greenspace_site')

Reading layer `greenspace_site' from data source 
  `/cloud/project/data/ordnance-survey/os-open-greenspace-gb.gpkg' using driver `GPKG'
Simple feature collection with 150149 features and 6 fields
Geometry type: MULTIPOLYGON
Dimension:     XY
Bounding box:  xmin: 9819.84 ymin: 8274.57 xmax: 655229.5 ymax: 1214133
Projected CRS: OSGB 1936 / British National Grid

Spatially subset data frame

Using coordinate-based indexer to spatially subset by bounding box (BBOX).

# Subset the data by intersection with BBOX

# Greater London bounding box
bbox <- st_bbox(c(xmin = 503568.1996, 
                  xmax = 561957.4962, 
                  ymin = 155850.7975, 
                  ymax = 200933.9026), 
                crs = st_crs(27700))

# Convert into a 'geometry'
bbox <- st_as_sfc(bbox)

# Spatial subsetting/filtering
osogs_filtered <- osogs[bbox, ]

OS Maps API ZXY resource

# Reproject to Web Mercator (EPSG: 3857)
osogs_filtered <- st_transform(osogs_filtered, crs = 3857)

# Basic plot using tmap
p <- tm_shape(osogs_filtered) +
  tm_fill(col = '#00cd6c') +
  tm_layout(frame = FALSE)

# OS Maps API layer
# Example uses Light Style in Web Mercator (EPSG:3857) projection
layer <- 'Light_3857'

# OS Data Hub project API key
key <- '7UTXMMWsGjjIzcBmLdAGnMO6WEAQi9Ng'

# Define the tile server parameters for the basemap
url <- paste0('https://api.os.uk/maps/raster/v1/zxy/', layer,
              '/{z}/{x}/{y}.png?key=', key)

Create custom interactive map

# Change plotting mode from static to interactive
tmap_mode('view')

tmap mode set to interactive viewing

# Combine the features and base map
final_plot <- p +
                tm_basemap(server = url)

final_plot

# Change back to the static plotting mode
tmap_mode('plot')

tmap mode set to plotting

Alternative method using leaflet

Demonstration using a different R package.

library(leaflet)
library(dplyr)

Attaching package: ‘dplyr’

The following objects are masked from ‘package:stats’:

    filter, lag

The following objects are masked from ‘package:base’:

    intersect, setdiff, setequal, union

Create custom map

# m is the map object all other layers will be added to.
m <- leaflet() %>%
  setView(lng = -0.105, lat = 51.507, zoom = 12)

m <- m %>% 
  addTiles(urlTemplate = url,
           attribution = paste0('Contains OS data © Crown copyright and database right ', 
                                format(Sys.Date(), "%Y")),
           group = 'OS Maps') %>%
  addPolygons(data = st_transform(osogs_filtered, 4326),
              color = '#00cd6c')

# Return map
m